Electrostatic speaker



April 26, 1960 'r. LINDENBERG mc'mosm'rxc SPEAKER 3 Sheets-Sheet 1 FiledNov. 9, 1955 E4 INVENTOR ATTORNEY T/wga'm [trait/thy mi,

T. LINDENBERG ELECTROSTATIC SPEAKER April 26, 1960 3 Sheets-Sheet 2 fiied Nov. 9, E1955 I NVENTOR v7210:1012 [mamhg ATTO RN EYS April 26, 1960T. LINDENBERG ELECTROSTATIC SPEAKER 3 Sheets-Sheet 3 Filed Nov. 9, 1955v I.NVENTOR 77110110 Lmdmhg BY m WM 21/ ATTORNEYS United States PatentELECTROSTATIC SPEAKER Theodore Lindenberg, West Islip, N.Y., assignor toPickering Associates, Inc., Oceanside, N.Y., a corporation of New YorkApplication November 9, 1955, Serial No. 545,875

14 Claims. (Cl. 179-111) This invention which is in part a continuationof my application, Serial No. 476,644 entitled Electrostatic Speakerwhich was filed in the United States Patent Office on December 21,generally to transducers for translating electrical waves into soundwaves or the reverse and more particularly relates to an electrostaticspeaker.

A speaker is a device which translates electrical signals applied to itinto sound waves. This translation is effected by the action ofelectrical signals on an acoustical element which generates sound wavesin response to the electrical signals. In an electrostatic speaker, theacoustical element is a diaphragm which forms a condenser plate and actswith a single fixed plate, or in some designs with a plurality of fixedplates, to form a condenser which serves as the load or output unit ofan electrical system. Variations in the output wave of the electricalsystem will then cause the diaphragm condenser plate to move insynchronism therewith since condenser plates are attracted or repelledas a function of electrical potential as well as of spacing anddielectric substance.

In the past, the advantages of an electrostatic speaker have beenrecognized and particular forms have come into use. The electrostaticspeakers used have been either single-sided in which forces act on oneside only of the vibratory diaphragm or push-pull in which theelectrostatic forces act on both sides of the vibratory diaphragm. Thepresent invention is described in terms of a push-pull typeelectrostatic speaker. However, the inventive concept herein presentedcomprehends an improved electrostatic transducer whether it be receiveror speaker of the single-side type or the push-pull type.

A chief advantage of an electrostatic speaker is that the force drivingthe diaphragm, being electrostatic, can be if the mounting is suitable,applied uniformly over its entire area. The electrostatic speaker istherefore free from the distortion which is present in speakers whichhave portions of the diaphragm vibrating out of synchronism. Anotherdistinctive feature of the performance of electrostatic transducers isthat the electrical signal energy acts directly on the diaphragm toeffect vibrations thereof and no intermediate transducer or conversionelement is necessary.

The electrostatic speaker as known heretofore suffers, however, fromcertain disadvantages which limit its use. Thus, the diaphragm or theair gap itself had usually been relied on to provide the protectiveinsulation against electrical breakdown. This protection was ofteninadequate and limits were thereby imposed on the voltages that could beused and on the specific power output of the device. Also, due toelectrical characteristics, the driving force often was not applied tothe acoustical ele ment uniformly and the frequency of response of thedevice was not uniform throughout.

Often close spacings, a film of trapped air, stiff diaphragm materialsand inadequate methods of mounting the diaphragm combined to restrict tovery small amplitudes the allowable diaphragm motion, required high1954, now abandoned, relates 2,934,61 l Patented Apr. 26, 1960 "ice.

2 mass diaphragms to be used and prevented the moving parts fromoperating in synchronism with one an other.

These disadvantages had confined the electrostatic speaker in practiceto use for reproducing a small range of very high audio frequencieswhere the output power requirement was least.

The invention herein disclosed has as its main object the provision ofan improved electrostatic speaker having increased sensitivity andfidelity of response over a wide range of frequencies.

Another object of this invention is to provide an improved electrostaticspeaker assembly having a high dielectric breakdown potential, in whichthe driving force is applied to the acoustical element uniformly and inwhich the diaphragm will have uniform response at all frequencies.

Still another object of'the invention is to provide an improved mountingassembly for the diaphragm of an electrostatic speaker which will permitthe use of a low mass diaphragm as the vibratory acoustical element,permit the diaphragm to vibrate at large amplitudes, pro vide uniformspacing between the diaphragm and a fixed condenser member and whichwill allow the moving parts thereof to move in synchronism with oneanother.

A further object of the invention is to provide an im provedelectrostatic speaker assembly including circuitry which will allow thespeaker to be utilized as the sole acoustic output of a system or incombination with multiple acoustic system outputs.

These and other objects are accomplished by furnishing an electrostaticspeaker assembly in which the diaphragm (a conductive, flexible, thin,member of low mass and stiffness) is supported between two electrodes inthe form of woven conductive metal screens or perforated plates whichare coated on the sides facing the diaphragm with resilient bristles(flocking) of non-conductive material in such a manner that the spacingbetween each outer electrode and the diaphragm is uniform when thediaphragm is static, and of such assembly that when the diaphragm isdrawn toward one electrode by an unbalance of charge on it in relationto the other electrode, the flocking will permit motion of the diaphragmin a linear manner as though many small springs were being compressed onthat side while expanding on the opposite side.

The construction of the speaker and manner of using the same isdescribed herein with reference to the drawings in which:

Fig. 1 is a partially sectional front elevational View of a speakerconstructed in accordance with the teachings of this invention;

Fig. 2 is a partially sectional plan view of the speaker shown in Fig.1;

Fig. 3 is a sectional elevation taken along the line'3-3 in thedirection of the arrows, indicated in Fig. 1;

Fig. 4 is a detailed view of portions of two flocked screens supportinga diaphragm between them;

Fig. 5 is an exploded view showing in perspective the cooperation of twostationary screens and the vibratory acoustical element;

Fig. 6 is a perspective view of the speaker frame;

Fig. 7. is a detailed view of the construction of the vibratoryacoustical element;

Fig. 8 is a diagrammatic illustration of one form of circuit arrangementfor operating an electrostatic speaker embodying this invention;

Fig. 9 is a schematic diagram showing the circuit which I is used inorder to couple an electrostatic speaker to the Fig. 11 illustrates theconnections on a plug which can cooperate with the cross-over receptaclein Fig. 9 to allow cross-over at a low frequency;

Fig. 12 is a schematic diagram of the connections used for thetransformer shown in Fig. 9 in order to couple quadruple electrostaticspeakers to the output circuit of a conventional amplifier;

Fig. 13 is a schematic diagram of the connections used for thetransformer shown in Fig. 9 in order to couple a single electrostaticspeaker to the output circuit of a conventional amplifier;

Fig. 14 is a schematic diagram of the connections used for thetransformer shown in Fig. 9 in order to couple eight electrostaticspeakers to the output circuit of a conventional amplifier; and

Fig. 15 is a schematic diagram of the connections used for thetransformer shown in Fig. 9 in order to couple twelve electrostaticspeakers to the output circuit of a conventional amplifier.

Construction of speaker The electrostatic speaker shown diagrammaticallyin the figures consists of a frame which is designated generally as 20and has thin wooden sections 21 and 22 in the form of identical segmentsof a circle which form a top and base respectively. Wooden beams 23, 24and 25 are of rectangular cross section and are each attached at one endto top 21 and at the other end to base 22 in such manner that the topand bottom are held rigidly parallel to one another. Top 21 has grooves26 and 27 therein, containing slots 28 and 29 respectively. Center beam24 has placed thereon a thin layer of foam rubber rib 30. This mbberrib, as will be seen later, absorbs vibration and isolates the center ofscreen 31 which is placed upon it. Screen 31 is one of two metal screens31 and 32 which are identical and of such construction as to beacoustically transparent. These screens are preferably made of wovengalvanized iron wire having, for example, about 15 wires to the inch,the wire size being about .010 inch, so as to provide relatively largeopenings to give acoustical transparency. Each of the screens is flockedwith fibers of cotton, rayon, fiber glass or other dielectric material.The flocking on screen 31 is designated generally as 33 while theflocking on screen 32 is designated generally as 34.

Flocked screens have been known in the past but they have been used forquite different purposes, one common use being as the cover for theopening in front of the speaker of a radio or phonograph. The techniqueof flocking a screen is well known and the flocking may be accomplishedelectrostatically or by mechanical vibration. In each of thesetechniques the wire of the screen is first coated with an enamel or thelike which is in a liquid or semi-plastic state.

Electrostatically flocking the screen consists of spraying fibers ofuniform length on the enamel, before it hardens, from a gun whilemaintaining a potential difference in the order of 10,000 volts betweenthe gun and the screen. The electrostatic action causes the fibers tostand largely on end on the screen forming a nap. They attach themselvesto the enamel coating and when the enamel hardens, they are permanentlyheld to the screen as a myriad of short bristles projecting to asubstantially uniform extent from the screen and generally normal to itsplane. If the mechanical technique is used for flocking, the screen ismechanically vibrated as the fibers are sprayed or dropped upon thecoated enamel. The preferred method is the electrostatic method sincethe individual flocked hairs stand normal to the screen when the enamelhardens, whereas if the mechanical technique is used, the individual.flocked hairs lay at different angles to the screen. However, in themechanical method a more even distribution of flocking is obtained.Although techniques 9 Obt in e ks-d. scree are tioned herein, I do notexclude other techniques.

The term flocked screens as used in this specification is meant todefine a screen prepared in accordance with this description in whichthe flocking on the screen is of uniform dimension, a portion of each ofthe fibres engaging the screen, and each fibre is separate from theother.

Disposed about the periphery of screen 31 and glued to the free ends ofthe flocks 33 thereon is a rubber gasket strip 35. Gasket 35 ispreferably no thicker than the thickness of two fiber lengths added tothe thickness of one diaphragm assembly which is situated betweenscreens 31 and 32 and designated generally in the draw ings as 36. Thediaphragm should be made of a very light, thin, flexible film having ahigh strength to mass ratio and both the low mass and the low stiffnessof the diaphragm are negligible factors in determining the resistance ofthe diaphragm to displacement in the flocked mounting and its resonantfrequency. Thus, the diaphragm may be made of a metalized film ofsynthetic resin and for this purpose I prefer to use a film made ofmylar polyester resin having a thin metallic coating. Diaphragm assembly36 consists of two films of dielectric material 37 and 38 preferablyfilms of mylar polyester resin each having a thickness of .00025 inch.Mylar film 38 is coated with aluminum or other electrically conductivematerial 39 on one of its surfaces. The conductive coating is applied byany well known metallizing technique and is preferably of a thickness ofthe order of 1 micro-inch. Aluminized face 39 is adjacent mylar film 37and film 37, which is a bit larger than film 38, has its edgesoverlapped about the non-aluminized side of film 38 thereby envelopingthis piece. Electric conductor 40, which is shown extending from thediaphragm assembly, is in contact with aluminum face 39 and serves as ameans whereby voltage may be applied to the aluminum coating as will beexplained later.

The diaphragm assembly 36 provides the vibratory condenser plate whichis the acoustically effective element of the speaker. The mylar servesto insulate the conducting or aluminized face 39 and to preventelectrical leakage since this film adds dielectric strength to thatprovided by the air and by the flocking itself. The diaphragm could be aseparate layer of foil with or without adjoining films of insulatingmaterial, but the novel construction shown in which a thin coated filmserves as a dielectric element and an acoustic element is preferred.Mylar film 37 also may be omitted. However, it is desirable asinsulation because it permits the main tenance of a higher potentialdifference between the fixed plate and the vibratory plate.

The diaphragm assembly is positioned inside of gasket 35 and mounted onthe free ends of bristles 33 and 34. This construction leaves thediaphragm assembly inert and also tends to center the assembly due tothe presence of the flocking which supports the assembly. The flockingis in effective engagement with the vibratory diaphragm in the sense ofengaging it to space it and support it, whether the fibers be in directcontact with the metal layer or in contact with an insulating film suchas described above.

The flocking fibers press against opposite sides of the diaphragm sothat the diaphragm is resiliently supported in floating fashion midwaybetween the fixed screens. The resistance of the diaphragm todisplacement as well as its resonant frequency, can be varied andcontrolled by varying the relative spacing and stiffness of theflocking. Thus, increasing the relative number and stiffness of theflocking fibers increases the diaphragm resistance to displacement andraises the resonant frequency while decreasing the relative number andstiffness of the fibers results in descreasing the resistance todisplacement and lowering the resonant frequency. The flocking fibersshould be so spaced and should be of such stiffness as to maintain allportions of the diaphragm centered between the two fixed sc'reenplatesin the absence of an resistances 50 and 51.

electric potential and to return all portions of the diaphragm tocentered position after it is displaced in either direction and thepotential removed. In this manner a substantially uniform response freefrom distortion over a wide frequency range is obtained. Within theselimits, the relative number or spacing of flocked fibers may be greatlyvaried. 7

When a suitable biasing or polarizing potential is applied to thescreens and diaphragm, as described in detail at a later point, anelectrostatic attractive force is set up between the two screens 31 and32 and also between each of these screens and the diaphragm. The effectof this is that the motion of the diaphragm can then be resisted only bythe flocking since the fibers are pressed against the interposeddiaphragm which is thereby well supported over its entire area but insuch a way that the diaphragm -is still free to vibrate.

conductor 42, which is attached to screen 32, extend along member 21 ingroove 26 and through slot 28 and are then attached to terminal posts 43and 44 respectively of terminal block 45 which is fastened to top 21inside the frame. Likewise, conductor 40 extends in groove 27 in top 21through slot 29 and is attached to a third terminal post 46 of terminalblock 45. The potential may be readily applied to the screens and thediaphragm through the terminal posts.

An acoustically transparent, decorative cloth 46' is wrapped over outerscreen 32 and attached to frame 29 by nails 47 and 48.

The term acoustically transparent as used herein is intended to define amaterial through which sound waves or waves audible to the human car canpass with ease and with a minimum of attenuation.

Circuit description when my speaker is to be the sole system output andincludes a conventional form of push-pull audio amplifier stage which isdesignated by the numberal 46' wherein the plates and cathodes of thetubes are shown only, which delivers the signal voltages E and E ofsubstantially equal magnitude but of opposite instantaneous polarity,being l80out of phase within usual limits of adjustment. These twovoltages representing the signal are applied to the fixed plates 31 and32 through condensers 47', 48', 49'; the voltage E from one side beingap plied to each of the fixed plates 31 and 32 and the volttage E fromthe other side being applied to the diaphragm v36.

The D.C. polarizing voltages E and E, are of opposite polarity and ofequal magnitude, and are applied respec tively to the two fixed plates31 and 32 through high Illustrative magnitudes are 500 volts and500.volts, creating a potential difference of 1000 volts between thefixed plates 31 and 32. The resistances are of the order of 2 to 5megohms to afford I a high degree of isolation of the A.C. signal fromthe power supply. The current is thereby so small that there is nomaterial absorption of the signal current.

The diaphragm 36 can be connected through a high resistance 52 to themidpoint of the D.C. supply, which relatively is a ground, so as tocreate an equal potential diiference and equal electrostatic forcebetween it and each of the fixed plates when the plates are equallyspaced; but since it is difficult to assure equal spacing of the plates,I prefer also to connect the diaphragm 36 to a variable tap 53 on aresistance 54 connected across the fixed plates as shown, so that thepotential differences between the diaphragm 36 and the two fixed platescan be adjusted and made unequal to compensate substantially for smalldifferences in the spacing of the diaphragm from the fixed plates. Theeffect of such adjustment is to establish more nearly equalelectrostatic forces on the diaphragm so that it is subject to nosubstantial unbalanced force tending to displace it before the signalvoltage is applied.

With the electrostatic system in substantial balance under the influenceof the polarizing voltages, application of the signal voltage E to thefixed plates and of E to metal layer 39 on diaphragm 36 causes what ineffect is a modulation of the polarizing voltages and of theelectrostatic forces acting on the diaphragm. Since the push-pullvoltages E and B are out of phase, the modulation effect is a multipleof the change in E and E individually, and in consequence the speakerhas greater sensitivity. For example, if the peak signal voltage is 50,then when E is +50 and E is -50, the eflect on the inter-plate potentialdifferences is as follows, in a case where the polarizing voltages areequal:

Individual Voltages Differential Plate 3 11 M u u u n u u n ResultantThe same resultant voltage values are found, but at the opposite fixedplates, when the signal voltage goes negative on those plates. Thedifferentials likewise are of the same magnitude but are reversed, 31-36being 400 and 32--36 being 600.

Thus the modulation effect of the signal voltage is to bring about anunbalance of the potential differences which create an unbalancedelectrostatic force tending to move the diaphragm, the unbalancereversing as the signal voltage reverses in polarity. In the exampleabove, at peak signal voltage, the diaphragm is subject to the drivingforce of a 200 volt unbalance of the potential differences, or afour-fold multiplication of the peak signal voltage. 1

The electrical circuit shown in Fig. 9 is also designed for use with thespeaker heretofore described. The circuit, which serves to supply directcurrent polarizing voltage to the electrostatic speaker in the properrelationship to the applied audio frequency voltages to produce themaximum speaker sensitivity with lowest distortion, also provides ameans of electrically coupling my improved tweeter to conventionalexisting systems.

In conventional systems the woofers have been found to be satisfactory,whereas heretofore it has been the tweeters which have proved-themselvesunsatisfactory. Because of this, the system shown in 'Fig. 9 is designedso that an electrostatic speaker constructed in accordance with theteachings herein, can be used specifically as a tweeter in combinationwith a system employing a conventional woofer. Thus, a unit may have aconventional woofer plus my tweeter. Also, by use of the presentcircuitry more than one tweeter may be coupled to the output of aconventional system in order to give greater acoustic loudness.

In Fig. 9, numerals 55 and 56 designate terminals to which are appliedAC. power such as that available from an ordinary wall socket, forexample, volts at 60 cycles. This voltage is applied to transformer 66and rectifying tube 58 which converts the A.C. voltage to D.C. voltagein the conventional manner. Condensers 59 and 60 in combination withresistor 61, form a filter which decreases the ripple on the rectifiedD.C. voltage while resistor 62 is a conventionalbleeder resistor.

Terminal 63 is connected to post 46 which is shown in Fig; 1 andsupplies positive polarity voltage to diaphragm 36 through conductor 40.Terminals 64 and 65 which are connected to posts 43 and 44 respectively,shown in Fig. 1, place the secondary of transformer 66 across fixedplates 31 and 32. The secondary of trans former 66 is grounded at centertap 67.

Transformer 66, depending upon connection and design, can match one ormore of my tweeters to the regular speaker output line of a conventionaloutput amplifier. The turns ratio of step-up transformer 66 is changedfor each case so that the output amplifier in use is loaded byapproximately the same capacitance for one or more tweeters. As shown inFig. 9, transformer 66 i arranged to match a single tweeter to theconventional output amplifier. The connections to the primary oftransformer 66 are made at receptacle 68 and the manner of connectingtransformer 66 for various matching conditions will 'be described below.

The output audio frequency voltage of a conventional output amplifier isapplied to the circuit shown in Fig. 9 at 69 and 70. The circuit shownprovides a means to pass the low frequency audio currents from an outputamplifier to a woofer connected at 71 and 72 while passing the highfrequency audio currents to transformer 66 and thence to the tweeterconnected at 64 and 65. The frequency at which this division of currenttakes place (known as the crossover frequency) can be changed in thiscircuit, according to the total area of the tweeters in use. Thecross-over frequency for use with a single tweeter is in the region of1200 to 1400 cycles per second and for quadruple tweeters is in theregion of 350-550 cycles per second. These figures are representativeonly and the actual choice of frequencies can be made over a wide range.

The connections used in order to accomplish the cross-over are made bythe insertion of plug 73 which is shown in Fig. in receptacle 68 when asingle tweeter is used. When this is done, terminals 74- through 79 matewith terminals 74a through 79a respectively. Terminals 74 and 75 arethereby connected by conductor 80, terminals 77 and 78 are connected byconductor 81 and terminals 77 and 79 are connected by conductor 32.

If quadruple tweeters are to be used, a plug 83 which is shown in Fig.11 is inserted in receptacle 68 rather than plug 73, thereby matingterminals 74 through 79 with terminals 74b through 795 respectively.This connects terminals 76 and 77 by conductor 84 and terminals 74 and78 by conductor 85. Fig. 13 shows schematically the manner in which theprimary of transformer 66 is connected when plug 73 is inserted inreceptacle 68, while Fig. 12 shows the connections of the primary whenplug 83 is used.

The cross over is accomplished by use of a cross-over network in thesystem. One leg of the cross-over network is composed of inductor 86 andcondenser 87 in series and the other leg of the cross-over networkconsists of condenser 88 in series with the primary of transformer 66.These legs are parallel and the woofer is connected across condenser 87,while the tweeter is connected to the secondary of transformer 66. Thus,the input voltage to the woofer is developed across condenser S7 and theinput voltage to the tweeter is developed at the secondary oftransformer 66. Since condenser 87 has greater impedance at lowerfrequencies, the greatest amplitude "oltage input to the woofer will beat low audio frequencies, while the transformer 66 will have developedat it secondary the greatest ampli tude voltage when a high frequencyaudio signal is received. The circuit, therefore, is selective andswitches high audio frequency currents to the tweeter and low audiofrequency currents to the woofer.

Figs. 14 and show schematically other transformer connections which maybe utilized to connect a number Qf tweeters to aconven tional outputamplifier. Fig 14 shows primary and secondary transformer connections 8which enable eight tweeters to be connected to .a single conventionalamplifier output, while Fig. 15 shows connections which enable twelvetweeters to be utilized. Windings in either case are designed to presentto the conventional amplifier the same capacitive reactance therebymaintaining the same load on that amplifier.

The connections in Fig. 14 are such that leads 89 and 9t) serve as theinput to transformer 66 as did leads 91b and 94b in Fig. 12. Lead 95 isconnected to a fixed plate of each of four tweeters and lead 96 isconnected to the other fixed plate of each of the tweeters. Lead 97 isgrounded. Leads 95' and 96 are connectedin like manner to the fixedplates of four other tweeters and lead 97' is grounded.

If it is desired to connect twelve tweeters, then the transformercircuitry shown in Fig. 15 is used, wherein leads 98 and 99 serve as theinput leads to transformer 66 as did leads 91b and 94b in Fig. 12. Lead100 is connected to a fixed plate of each of four tweeters and lead 101is connected to the other fixed plate of each of the tweeters. Lead 162is grounded. Lead 100 and 161 are connected in like manner to the fixedplates of four other tweeters and lead 102' is grounded. Likewise, leads100", 101 and 102 are connected in the same manner as are leads 101, and102' respectively.

Also included in the circuit shown in Fig. 9 is provision for variableattenuation of either the low frequency currents or the high frequencycurrents in order to obtain a balance of acoustic loudness between thewoofer and tweeter or between sets of woofers and tweeters. This isaccomplished through use of a variable resistor 103 which can only beput in one leg of the cross-over network at a time due to switch 104.Thus, the attenuator is arranged so that it can be introduced intoeither the low frequency currents or the high frequency currents but notinto both at the same time. When the tweeter is coupled to the circuitshown in Fig. 9, lead 63 is connected to conductor 40 at post 46 therebyapplying positive DC. voltage to diaphragm 36. A signal voltage from anoutput amplifier is received by the circut shown in Fig. 9 at terminals69 and 70. Due to operation of the crossover network, low frequencysignals are applied at terminals 71 and 72 which are attached to awoofer, while high frequency signals are applied at terminals 64 and 65which are attached to leads 41 and 42 respectively. When high frequencysignal voltages are received they place charges on fixed plates 31 and32. The effect of this i to apply electrostatic forces on diaphragm 36alternately in one direction and then in the other as the high frequencysignal voltage alternates in polarity. Due to the push-pull arrangementdescribed here, which is the preferred embodiment, the diaphragm isdriven positively in both directions by an alternating force rather thanbeing subject to merely unidirectional electrostatic force which resultswhen a single sided tweeter is used.

As the diaphragm i displaced in either direction from its mid-position,its decreased separation from the fixed plate it is approaching causesan increase in its attraction to that plate. A correspondingly increasedseparation from the other plate weakens the effect upon it of thatplate. The resultant of these changes in the electrostatic forcesincident to displacement of the diaphragm is to augment the effect ofthe potential differences which create the driving force.

While the invention is here described and claimed in terms of a speaker,the invention is useful also as a microphone or other transducer totranslate sound waves into an electrical wave. I include this within thescope of the claims. As so used, the polarizing mean would beessentially the same, but the circuit here shown for applying a signalvoltage would be replaced by any suitable circuit for taking off thecurrent and applying it, for example, to the grid of an amplifier.

Thus, among others, the several objects of the invention as specificallyaforenoted, are achieved. Obviously, numerous changes in constructionand rearrangement of parts might be resorted to without departing fromthe spirit of the invention as defined by the claims.

I claim:

1. An electrostatic speaker comprising in combination a frame, arelatively fixed plate formed of an electrically conductive materialmounted on said frame, nonconductive bristles of uniform length eachrigidly attached at one end to said plate and separate from each other;an acoustic element also formed of an electrically conductive materialin effective engagement with the free end of each of said bristles; andmeans to electrostatically charge said plate and said acoustic elementto establish an electrostatic force between said plate and said acousticelement.

2. An electrostatic speaker in accordance with claim 1 in which thecaustic element is afilm of dielectric material having on a side anadherent coating of electrically conductive material. r

3. An electrostatic speaker in accordance with claim 1 in which theacoustic element is a film of metalized polyester resin.

4. An electrostatic speaker in accordance with claim 1 in which thefixed plate is an acoustically transparent member of electricallyconductive material.

5. An electrostatic speaker in accordance with claim 1 in which thefixed plate is an acoustically transparent screen of electricallyconductive material.

6. An electrostatic speaker in accordance with claim 1 in which thenon-conductive bristles are fiber glass threads.

7. An electrostatic speaker comprising in combination a frame, arelatively fixed plate formed of an electrically conductive materialmounted on said frame, a surface of said plate, a second plate formed ofan electrically conductive material mounted on said frame, a secondsurface of said second plate spaced from and parallel with said firstsurface, non-conductive bristles of uniform length each rigidly attachedat one end to said first surface and separated from each other, secondnon-conductive bristles of uniform length each rigidly attached at oneend to said second surface and separated from each other, an acousticelement also formed of an electrically conductive material with onesurface thereof in effective engagement with the remaining ends of saidfirst bristles and the remaining surface thereof in effective engagementwith the remaining ends of said second bristles, and means toelectrostatically charge said plates and said acoustic element toestablish an electrostatic force between said plates and acousticelement.

8. An electrostatic speaker in accordance with claim 7,

in which the acoustic element is a film of dielectric material having ona side an adherent coating of electrically conductive material.

9. An electrostatic speaker in accordance with claim 7, in which theacoustic element is a film of metallized polyester resin.

10. An electrostatic speaker in accordance with claim 7, in which thefixed plates are acoustically transparent members of electricallyconductive material.

11. An electrostatic speaker in accordance with claim 7, in which thefixed plates are acoustically transparent screens of electricallyconductive material.

12. An electrostatic speaker in accordance with claim 7, in which thenon-conductive bristles are fiber glass threads.

13. An electrostatic speaker comprising in combination an electricallyconducting diaphragm, parallel fixed acoustically transparentelectrically conducting members positioned on opposite sides thereofwith a surface of one of said electrically conducting members and asecond surface of the other of said electrically conducting membersspaced from and facing said diaphragm, non-conducting resilient supportsrigidly attached to each of said surfaces, each of said resilientsupports having one portion thereof rigidly attached to one of saidsurfaces and a second portion thereof engaging said electricallyconducting diaphragm, said non-conducting resilient support supportingthe diaphragm over substantially the whole of its area.

14. An electrostatic speaker in accordance with claim 13, in which theresilient supports constitute a plurality of fibers, and all of thefibers are uniform in dimension and each of the fibres is separate fromthe others.

References Cited in the file of this patent UNITED STATES PATENTS1,622,039 Lee Mar. 22, 1927 1,762,981 Hartley June 10, 1930 1,764,008Crozier June 17, 1930 1,978,200 Heising Oct. 23, 1934 2,520,798 De BoerAug. 29, 1950 2,681,446 Erwing et al. June 15, 1954 2,686,847 AamadtAug. 17, 1954 2,824,178 Bobb Feb. 18, 1958 FOREIGN PATENTS 695,243France Dec. 12, 1930 1,083,302 France Jan. 7, 1955

